They talked about seahorses on QI this season:
(It starts at 5:47 – click here if you don’t feel like fast-forwarding through fairly gross and totally irrelevant other things.)
[ Sorry, Youtube took it down. ]
Again, it’s at 5:47.
Seahorses have a very rare mating system in which the male has a brood pouch into which the female implants the egg. To the best of my knowledge, it’s the only species in which the males give birth to live young.
So David Mitchell says, “I know it is more complicated than I think, but, sort of, if it’s the one that’s giving birth, why don’t we call that the female? What’s the female one do that’s more female than giving birth?”
Stephen Lovely Fry says, “She plants the eggs in the male.”
David Mitchell: “Oh right. Why aren’t they actually large sperm? Why are they defined as eggs? They don’t come from the one that gives birth. It just seems so abitrary!” And he goes on, entertaingly.
Which is hilarious &c but also quite interesting, right? Mr Mitchell is a born philosopher of science. See, look:
His working definition of “female” is “the one that gives birth,” which is a totally reasonable definition for a mammal to have, since without exception in our class of vertebrates it’s true.
But the more universal definition of “female” (and I’m not going to make the mistake of insisting that a more universal definition is in any way useful or important day-to-day for most mammals, such as Mr Mitchell; it’s simply more universal without being in any way superior) is “the one with the eggs,” as he correctly deduces.
Why? What actually is the difference between eggs and sperm? Three things:
Sperm are little and many. They may result in the same overall metabolic investment as the conspecific’s eggs, but sperm are small in size and large in number. In humans, if the egg were the size of a fridge, the sperm would be very roughly the size of a corndog.
Sperm travel independently. Even in seahorses, where you might assume – indeed, scientists did assume for a long time – that fertilization would happen internally after the “impregnation” of the male, it turns out the sperm are ejaculated into the sea and have to travel back into the brood pouch in order to fertilize the eggs. Which brings us to…
Sperm compete with each other to fertilize eggs. Because they can travel and there are more of them, sperm compete to fertilize eggs. Thus a smaller proportion of sperm are represented in the next generation, compared to eggs. This is often paralleled in the reproductive success of individuals in a species; a larger proportion of females make it into the next generation than males, among whom competition if fiercer and, not coincidentally, winners are fewer. Each of us humans has more female ancestors than male ancestors; you might think we’d have to have an equal number of both, but no.
So males are the ones with the large number of tiny, traveling gametes.
Is that a better definition than “the one that gives birth”? Well, insofar as there are species in which neither gives birth – like in fish species where sperm and eggs are simply deposited on the sea/ocean/river floor and left to fend for themselves – it’s important not to use birth as the determining characteristic.
So we use gametes as the universal. And in those species where the gametes are not different, we simply don’t assign sexes; but the sperm/egg divide is nice and reliable for lots of species. Oh and there are also species with males and hermaphrodites (androdioecy) or females and hermaphrodites (gynodioecy).
In short, sex is complicated and various, but it boils down to numerous small mobile gametes (males) fighting to fertilize a smaller number of gigantic, stationary gametes (females). And that’s why we count the seahorses who get pregnant as males.